1. Field of the Invention
The invention relates generally to the field of electron microscopes. More particularly, the invention relates to transmission electron microscopes. Specifically, a preferred implementation of the invention relates to electron holography microscopes.
2. Discussion of the Related Art
A hologram is an interference pattern that is directly related to the amplitudes and phases of waves, created by the overlapping of an image- and reference- wave. Because of this, an electron hologram is ideal for purposes of quantitative mapping of electrostatic fields mean inner potentials and magnetic fields in and outside of materials, as discussed in detail in xe2x80x9cIntroduction to Electron Holographyxe2x80x9d, Editors Edgar Voelkl et al3.
Currently, the common methodology for creating electron holograms is to add an electron biprism to the Transmission Electron Microscope (TEM). In most cases, the biprism is situated behind the back focal plane of the objective lens, perpendicular to the electron beam so that it can split the field of view. A specimen is placed in one side of the visual field so that the other half of the visual field remains empty. Thus half of the emitted electron beam contains the image and the other half is the reference beam, as shown in FIG. 22. The electron beam travels through the condenser lens or condenser lens system 200, shines on the specimen 107, and is magnified by the objective lens 108 before passing the biprism 109 and being separated into two beams. When no voltage is applied to the biprism 109, as shown in FIG. 2A, the beams remain separate. Once a voltage has been applied to the biprism (in this setting a positive voltage), the electron waves on both sides of the biprism are bent toward the center until they overlap to create an interference pattern of parallel fringes, as shown in FIG. 2B.
In addition, the condenser system may be excited such that it forms an elongated shape rather than a circular shape. The long axis of the oval is perpendicular to the electron beam and the biprism in order to improve the contrast of the interference fringes. This detail is also discussed in xe2x80x9cIntroduction to Electron Holography.xe2x80x9d3 
Several TEM models use this method to offer the ability to generate electron holograms as one of their functions. In the Brookhaven National Laboratory model, JEOL-3000F, as shown in FIG. 11, the emitter 101 transmits an electron beam through the accelerator 102, which accelerates the beam. The beam then passes through the first and second condenser lenses, 105 and 106, which adjust intensity and illuminating area of the electron beam before it strikes the specimen 107. The resulting object beam containing an electron image is magnified by the objective lens 108 and encounters an electron biprism 109. The interference pattern is formed below the biprism. The electron image and interference pattern are magnified by the intermediate lens system 110 and the projector lens system 111. A final image can be observed in the viewing chamber 112.
For normal microscope operation, the intermediate lens system (sometimes the first intermediate lens is also called the diffraction lens, e.g., by Philips) as well as the projector lens systems are used to achieve a variable magnification ranging from approximately 4,000 to 2,000,000 times. The objective lens remains usually at a fixed magnification in order to maintain the performance level and image quality. However, it can be turned off for some specific applications as well as for achieving a low magnification of several hundred times. This is the state of the art for any conventional transmission electron microscope (TEM).
A problem with this technology to the present day has been that by introducing a biprism slightly above (or below) an image plane below the object introduces a new system requirement. In order to properly record a hologram, the interference fringes created by the biprism need to be magnified such that they can be recorded usefully on a recording device. If the magnification is too small, the interference fringes become too fine for recording, while over-magnification severely limits the available field of view. Thus, the intermediate and projector lens systems below the biprism are necessary to provide the correct magnification for the interference fringes from the biprism onto the final image or recording plane. As there is an optimum magnification value for interference fringes, the intermediate- and projector-lens systems provide a preset magnification and are no longer available for changing the magnification of the object, as they do in non-holographic modes.
The additional system requirement of electron holography, as caused by the position of the biprism has severe consequences for the use of the electron microscope. As the intermediate and projector lens system is used to provide optimum condition for imaging the interference fringes, the magnification factor from those lenses is fixed and thus only the objective lens remains to accommodate a change of magnification of the object. Obviously, the objective lens alone can not cover the magnification range required for a useful operation of a TEM as the available magnification range for the object becomes minimal.
The use of a TEM for imaging in the holographic mode is further restricted, as the entrance plane of the first lens after the biprism needs to remain approximately fixed; any change of the position of the entrance plane of the first lens below the biprism will affect the number and quality of the interference fringes. For an increase in the number of interference fringes the contrast of the interference fringes decreases and vice versa. Thus, there is an optimum position for the entrance plane of the first lens after the biprism. This makes it very difficult to change the magnification of the objective lens while maintaining the interference fringes unchanged and the object in focus. This leads to the situation that for most TEMs there are only two magnification modes available: xe2x80x9cmode onexe2x80x9d with the objective lens turned off and xe2x80x9cmode twoxe2x80x9d with the objective lens at its standard excitation. The two modes are well known in literature and called xe2x80x9clow resolution modexe2x80x9d and xe2x80x9chigh resolution mode.xe2x80x9d (Hitachi microscopes are an exception insofar as the biprism can be installed below the first intermediate lens, and thus there are two lenses available to adjust the magnification of the object while maintaining the interference fringes. However, this is a non-standard operation and severely decreases the overall performance of the instrument (for reasons not to be discussed here and of no consequence for this patent application).
Another problem arises from using a standard TEM for holographic imaging and concerns the second part of the patent application. The condenser system, sometimes partially integrated with the pre-field lens system of the objective lens has to provide an illumination that can handle the full magnification range. A condenser lens system is generally designed such that it provides a round, symmetric illumination at all magnifications. However, at high magnification values the roundness of the illumination has to be fine-tuned by a set of so-called stigmators. These stigmators compensate small deviations of the real condenser system from the ideal system and have practically no effect at low magnifications. In the special case where the objective lens is turned off, the effect of these stigmators is negligible. This means, for the low-resolution-mode with the objective lens off, the illumination is remains round and thus severely limits the use of holography: the number of interference fringes at a reasonable contrast and beam intensity is significantly smaller in the low-resolution-mode than for the high-resolution-mode.
As discussed above, there are several problems with using a standard TEM for basic, routine electron holography. First, using a standard TEM for holography reduces the available magnifications values of the instrument from over 50 to basically two values (for example 10,000 and 500,000 (magnification values vary with brand and type of microscope)). Second, the usability of TEMs for holography is further limited as the astigmatic illumination conditionxe2x80x94which allows for a significant improvement in the number of interference fringes at a reasonable contrastxe2x80x94is available in the high-resolution-mode only.
Many specimens that would be of crucial interest for scientists in materials sciences or biological sciences can presently not be investigated by means of electron holography. A solution is presented in this patent application that will allow adapting the magnification to the object while maintaining correct imaging and illumination conditions for the interference fringe pattern.
Heretofore, the requirements of greater magnification possibilities and a more general specimen acceptability for TEMs to produce electron holograms have not been fully met. What is needed is a solution that simultaneously addresses both of these requirements.
There is a need for the following embodiments. Of course, the invention is not limited to these embodiments.
According to an aspect of the invention, a method comprises: changing a size of an electron object image with a set of electron lenses; creating an interference pattern from the electron object image; and imaging interference fringes of the interference pattern onto an image plane, while not changing a magnification of the interference pattern.
According to another aspect of the invention, an apparatus comprises: a first set of electron lenses adapted to change an electron object image size; an electron biprism coupled to the first set of electron lenses; and a second set of electron lenses coupled to the electron biprism, the second set of electron lenses adapted to image interference fringes of an interference pattern created by the electron biprism onto an image plane without changing a magnification of the interference pattern.